The statements in this section merely provide background information related to the present disclosure, and accordingly such statements are not intended to constitute an admission of prior art.
Known internal combustion engines that are configured to operate lean of stoichiometry may be equipped with exhaust aftertreatment systems including three-way catalytic converters and ammonia-selective catalytic reduction (NH3-SCR) devices having catalysts to treat constituents in an exhaust gas feedstream. Such known systems generate and store ammonia on a catalytic surfaces in the NH3-SCR device that is employed to react with exhaust gases. Ammonia may be generated by passing a rich exhaust gas feedstream through a three-way catalytic converter upstream of an NH3-SCR device under specific engine operating conditions. The stored ammonia reduces nitrogen oxide (NOx) molecules in the exhaust gas feedstream to elemental nitrogen and water under a range of operating conditions associated with exhaust temperature and flowrate. Stored ammonia may separate from the catalyst in the NH3-SCR device and pass into the exhaust gas feedstream under specific operating conditions, such during engine acceleration. This is referred as ammonia slip.
A known method to prevent ammonia slip, i.e., expulsion of stored ammonia into the atmosphere, includes using a second oxidation catalyst downstream of the NH3-SCR device. Use of the second oxidation catalyst increases flow restriction in the exhaust gas feedstream, thus reducing maximum engine power and brake-specific fuel consumption. Additionally, use of the second oxidation catalyst increases vehicle mass and adds complexity to the exhaust aftertreatment system.